1. Field of the Invention
The present invention relates to an optical information medium where two optical information substrates are bonded together, and a method and an apparatus for fabricating the same.
2. Description of the Related Art
In order to enhance the density of an optical disk, reducing the wavelength of a reproduced laser beam and increasing the numerical aperture (NA) of an objective lens are required. However, increasing the NA of an objective lens makes the allowable tilt of a disk very small. For example, for a substrate with a thickness of 1.2 mm which is the thickness of CDs, the tilt of the substrate allowable for an objective lens with an NA of 0.6 is about 0.25.degree.. This is equal to the error occurring at the mounting of an optical head to a player, meaning that no allowance is left for the tilt of an optical disk due to a shape change of the disk. Such an optical disk is not practical.
The allowable range for the tilt of a disk can be widened by reducing the thickness of a substrate. Thus, a practical optical disk with high density used for an objective lens with high NA can be realized. For example, by reducing the thickness of the substrate to 0.6 mm which is a half of the thickness of CDs, the tilt allowable for an objective lens with an NA of 0.6 is as high as about 0.75.degree.. In this case, even if an error of 0.25.degree. arises at the mounting of an optical head to a player, a tilt of the disk of 0.5.degree. due to a shape change of the disk is allowed.
An optical disk composed of a single thin substrate tends to bend down by its own weight. To avoid this problem, it is recommended to bond two substrates together. This serves, not only to increase the mechanical strength, but also to double the capacity of the disk by using two sides for information recording.
FIG. 1 is a sectional view of a typical optical disk having two substrates bonded together with a radiation curable resin. Referring to FIG. 1, a first substrate 1 has a first information signal surface 2, on which a first reflection film 3 made of metal and the like including aluminum as a main component is formed. A second substrate 4 has a second information signal surface 5, on which a second reflection film 6 made of the same material as the first reflection film 3 is formed. A layer of a radiation curable resin 7 is formed between the first and second reflection films 3 and 6, which are facing each other, so as to bond the first and second substrates 1 and 4 together.
Referring to FIGS. 2A to 2D, a conventional method for fabricating such an optical disk will be described (Japanese Laid-Open Patent Publication No. 6-238846). The first substrate 1 having the first information signal surf ace 2 is formed with a transparent resin such as polycarbonate by injection molding and the like. The first reflection film 3 is then formed on the first information signal surface 2 of the first substrate 1 by sputtering, vapor deposition, and the like. Also, the second substrate 4 having the second information signal surface 5 is formed by injection molding and the like, and the second reflection film 6 is formed on the second information signal surface 5 of the second substrate 4 by sputtering and vapor deposition. The first and second reflection films 3 and 6 are made of metal including aluminum as a main component. Then, while the first substrate 1 being rotated at low speed, the radiation curable resin 7 is applied to the surface of the first substrate 1 on which the first reflection film 3 is formed, forming a donut-shaped resin layer (FIG. 2A). The second substrate 4 is then placed on the radiation curable resin 7 so that the second reflection film 6 on the second information signal surface 5 of the second substrate 4 faces the radiation curable resin 7 (FIG. 2B). The first and second substrates 1 and 4 are then rotated together at high speed to allow the radiation curable resin to spread in the space between the first and second substrates 1 and 4 (FIG. 2C). The radiation curable resin is irradiated with radioactive rays (UV rays in FIG. 2D) through the second substrate 4 and the second reflection film 6 formed thereon, to cure the radiation curable resin and thus to bond the two substrates integrally (FIG. 2D).
In this conventional example, each of the two bonded substrates has an information signal surface, and the reflection film is formed on each information signal surface. The reflection film allows a slight amount of radioactive rays to pass therethrough and thus causing the radiation curable resin to be cured. For example, when the reflection film is made of aluminum and irradiated with UV rays, the UV transmittance of the aluminum film is 1% or less, which is large enough to cure the resin sufficiently.
The disk with bonded thin substrates has such a large capacity that the capacity of only one side of the disk is enough for most software. Accordingly, one of the bonded substrates can be a transparent dummy substrate. In this case, the radiation curable resin can be rapidly and easily cured with radioactive rays passing through the transparent substrate (Japanese Laid-Open Patent Publication Nos. 5-63668 and 5-195011).
Alternatively, a semi-transparent film can be formed on one of the two substrates, and the two information signal surfaces can be accessed from one side of the disk. In this case, the radiation curable resin can be rapidly and easily cured with radioactive rays through the semi-transparent film.
As described above, in the above conventional method for fabricating a disk with two substrates bonded together, a radiation curable resin is applied to one of the substrates to form a donut-shaped resin layer while the substrate is being rotated at low speed, and the radiation curable resin is allowed to spread in a space between the two substrates while the two substrates are being rotated at high speed. In this case, when the high-speed rotation of the two substrates starts before the radiation curable resin spreads toward the inner circumferences of the substrates, the inner circumference portion of the disk is not supplied with a sufficient amount of resin. The resultant disk is inferior in strength. On the contrary, when the high-speed rotation of the two substrates starts only after the resin has sufficiently spread in the space between the two substrates, the resin tends to protrude to center holes of the substrates. This causes troubles such as decentering when the resultant disk is mounted on a turntable of a player.